Debris in the circulating coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable and may include broken fuel assembly fasteners.
The invention involves a spring retention cap for a nuclear fuel assembly end fitting leaf spring assembly which is designed to prevent the creation of debris, such as pieces of broken holddown spring retention screws. The cap of the invention also minimizes the loss of holddown force, the chance that a failure of one screw will impart a jacking force which leads to the failure of a second screw, and interference with control rod operation because of rotation of a leaf spring. Moreover, the cap is designed to provide simple installation and removal procedures. The cap also saves fuel reconstitution expense, because it eliminates the requirement of welds for preventing screw rotation.
More importantly, the unique cap provides the option of continuing to operate safely, without reconstitution, even if a holddown spring retention screw breaks.